Diseases such as AIDS and leukemia caused by retroviruses have intensified the need to understand the mechanisms of retrovirus replication. One of our objectives is to understand how retroviral cDNAs are integrated into the genome of infected cells. Because of their similarities to retroviruses, long terminal repeat (LTR)-retrotransposons are important models for retrovirus replication. The retrotransposon under study in our laboratory is the Tf1 element of the fission yeast Schizosaccharomyces pombe. We are particularly interested in Tf1 because its integration exhibits a strong preference for pol II promoters. This choice of target sites is similar to the strong integration preferences human immunodeficiency virus 1 (HIV-1) and murine leukemina virus (MLV) have for pol II transcription units. Currently, it is not clear how these viruses recognize their target sites. We therefore study the integration of Tf1 as a model system with which we hope to uncover mechanisms general to the selection of integration sites. An understanding of the mechanisms responsible for targeted integration could lead to new approaches for antiviral therapies and to improvements in the application of viral vectors in gene therapy. The extraordinary capacity of DNA sequencing can create ultra dense maps of integration that are being used to study the mechanisms that position integration. Unfortunately, the great increase in the numbers of insertion sites detected comes with the cost of not knowing which positions are rare targets and which sustain high numbers of insertions. To address this problem we developed the serial number system, a TE tagging method that measures the frequency of integration at single nucleotide positions. We sequenced 1 million insertions of retrotransposon Tf1 in the genome of S. pombe and obtained the first profile of integration with frequencies for each individual position. Integration levels at individual nucleotides varied over two orders of magnitude and revealed that sequence recognition plays a key role in positioning integration. The serial number system is a general method that can be applied to determine precise integration maps for retroviruses and gene therapy vectors. Transposable elements constitute a substantial fraction of the eukaryotic genome and as a result, have a complex relationship with their host that is both adversarial and dependent. To minimize damage to cellular genes TEs possess mechanisms that target integration to sequences of low importance. However, the retrotransposon Tf1 of Schizosaccharomyces pombe integrates with a surprising bias for promoter sequences of stress response genes. The clustering of integration in specific promoters suggests Tf1 possesses a targeting mechanism that is important for evolutionary adaptation to changes in environment. We found this year that Sap1, an essential DNA binding protein, plays an important role in Tf1 integration. A mutation in Sap1 resulted in a 10-fold drop in Tf1 transposition and measures of transposon intermediates supports the argument that the defect occurred in the process of integration. Published ChIP-Seq data of Sap1 binding combined with high-density maps of Tf1 integration that measure independent insertions at single nucleotide positions show that 73.4% of all integration occurred at genomic sequences bound by Sap1. This represents high selectivity since Sap1 binds just 6.8% of the genome. A genome-wide analysis of promoter sequences revealed that Sap1 binding and amounts of integration correlate strongly. More importantly, an alignment of the DNA binding motif of Sap1 revealed integration clustered on both sides of the motif and showed high levels specifically at positions +19 and -9. These data indicate that Sap1 contributes to the efficiency and position of Tf1 integration. Transposable elements (TEs) are common constituents of centromeres. However, it is not known what causes this relationship. Schizosaccharomyces japonicus contains 10 families of Long Terminal Repeat (LTR)-retrotransposons and these elements cluster in centromeres and telomeres. In the related yeast, Schizosaccharomyces pombe LTR-retrotransposons Tf1 and Tf2 are distributed in the promoter regions of RNA pol II transcribed genes. Sequence analysis of TEs indicates that Tj1 of S. japonicus is related to Tf1 and Tf2, and uses the same mechanism of self-primed reverse transcription. Thus, we wondered why these related retrotransposons localized in different regions of the genome. To characterize the integration behavior of Tj1 we expressed it in S. pombe. We found Tj1 was active and capable of generating de novo integration in the chromosomes of S. pombe. The expression of Tj1 is similar to Type C retroviruses in that a stop codon at the end of Gag must be present for efficient integration. 17 inserts were sequenced, 13 occurred within 12 bp upstream of tRNA genes and 3 occurred at other RNA pol III transcribed genes. The link between Tj1 integration and RNA pol III transcription is reminiscent of Ty3, an LTR-retrotransposon of Saccharomyces cerevisiae that interacts with TFIIIB and integrates upstream of tRNA genes. The integration of Tj1 upstream of tRNA genes and the centromeric clustering of tRNA genes in S. japonicus demonstrate that the clustering of this TE in centromere sequences is due to a unique pattern of integration. Retroviruses and Long Terminal Repeat (LTR)-retrotransposons have distinct patterns of integration sites. The oncogenic potential of retrovirus-based vectors used in gene therapy is dependent on the selection of integration sites. The LTR-retrotransposon Tf1 of Schizosaccharomyces pombe is studied as a model because it integrates into the promoters of stress response genes. Although integrases (INs) encoded by retroviruses and LTR-retrotransposons are responsible for catalyzing the insertion of cDNA into the host genome, it is thought that distinct host factors are required for the specificity of integration site selection. We tested this hypothesis with a genome-wide screen of host factors that promote Tf1 integration. By combining an assay for transposition with a unique genetic assay that measures cDNA present in the nucleus, we could identify factors that contribute to integration. We utilized this assay to test a collection of 3,004 S. pombe strains with single gene deletions. Using these screens and immunoblot measures of Tf1 proteins, we identified a total of 61 genes that promote integration. The integration factors participate in a range of processes including nuclear transport, transcription, mRNA processing, vesicle transport, chromatin structure and DNA repair. Surprisingly, a number of pathways we identified were found previously to promote integration of the LTR-retrotransposons Ty1 and Ty3 in Saccharomyces cerevisiae, indicating the contribution of host factors to integration are common in distantly related organisms. The DNA repair factors are of particular interest because they suggest the pathways that repair the single stranded gaps opposite integration sites are the same in diverse eukaryotes.